Diesel locomotives are already hybrids (pretty much all diesels built since the mid-20th century are diesel-electric). I imagine that a generator large enough to charge up a locomotive would add too much extra weight to the design, however I could see a modified diesel locomotive being utilized to both push a dead electric train and to at least partially recharge that train's batteries during the journey.

Diesel-electric drivetrains are not "hybrids" in the sense that they have an energy storage device. The same type of drivetrain was used in submarines and tanks during WWII, no one calls those a hybrid.